Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Such fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to complete the electrochemical reaction and form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to complete the electrochemical reaction and form liquid water as the reaction product.
In conventional fuel cells, the MEA is interposed between two fluid-impermeable, electrically conductive plates, commonly referred to as the anode and the cathode plates, respectively. The plates serve as current collectors, provide structural support for the porous, electrically conductive electrodes, provide means for carrying the fuel and oxidant to the anode and cathode, respectively, and provide means for removing water formed during operation of the fuel cell. When the channels are formed in the anode and cathode plates, the plates are normally referred to as fluid flow field plates. When the anode and cathode plates overlay channels formed in the anode and cathode porous material, the plates are normally referred to as separator plates.
Reactant feed manifolds are generally formed in the anode and cathode plates, as well as in the MEA, to direct the fuel (typically substantially pure hydrogen or hydrogen-containing reformate from the conversion of hydrocarbons such as methanol or natural gas) to the anode and the oxidant (typically substantially pure oxygen or oxygen-containing gas) to the cathode via the channels formed in either the fluid flow field plates or the electrodes themselves. Exhaust manifolds are also generally formed in the anode and cathode plates, as well as the MEA, to direct unreacted fuel and oxidant, as well as water accumulated at the cathode, from the fuel cell.
Multiple fuel cell assemblies comprising two or more anode plate/MEA/cathode plate combinations, referred to as a fuel cell stack, can be connected together in series (or in parallel) to increase the overall power output as required. In such stack arrangements, the cells are most often connected in series, wherein one side of a given fluid flow field or separator plate is the anode plate for one cell, the other side of the plate is the cathode plate for the adjacent cell, and so on.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designation, have been used effectively in electrochemical fuel cells. Fuel cells employing Nafion-type cation exchange membranes require accumulated water to be removed from the cathode (oxidant) side, both as a result of the water transported across the membrane with cations and product water formed at the cathode from the electrochemical reaction of hydrogen cations with oxygen. An experimental perfluorosulfonic ion exchange membrane, sold by Dow under the trade designation XUS 13204.10, appears to have significantly less water transported with hydrogen cations across the membrane. Fuel cells employing the Dow experimental membrane thus tend to accumulate less water on the cathode (oxidant) side, as the accumulated water at the cathode is essentially limited to product water formed from the electrochemical reaction of hydrogen and oxygen.
A typical prior art fluid flow field plate, exemplified by General Electric Company and Hamilton Standard in a 1984 report for the U.S. Department of Energy (LANL No. 9-X53-D6272-1), included a plurality of parallel open-faced fluid flow channels formed in a major surface of a rigid, electrically conductive plate. The parallel channels extended between an inlet header and an outlet header formed in the plate. The parallel channels were typically rectangular in cross-section, and about 0.03 inches deep and about 0.03 inches wide. The inlet header was connected to an opening in the plate through which a pressurized reactant (fuel or oxidant) stream is supplied. The outlet header was connected to an opening in the plate through which the exhaust stream is discharged from the cell. In operation, the reactant stream ran from the inlet to the inlet header and then to the parallel channels from which reactant from the stream diffused through the porous electrode material to the electrocatalytically active region of the MEA. The stream then flowed to the outlet header and then to the outlet from which it was exhausted from the fuel cell.
Watkins U.S. Pat. Nos. 4,988,583 and 5,108,849 issued Jan. 29, 1991 and Apr. 28, 1992, respectively, describe fluid flow field plates which include a fluid supply opening and a fluid exhaust opening formed in the plate surface. Continuous open-faced fluid flow channels formed in the surface of the plate traverse the central area of the plate surface in a plurality of passes, that is, in a serpentine manner. Each channel has a fluid inlet at one end and a fluid outlet at the other end. The fluid inlet and outlet of each channel are directly connected to the fluid supply opening and fluid exhaust opening, respectively. The continuous channel design promotes the forced movement of water through each channel before the water can coalesce, thereby promoting uniform reactant flow across the surface of the cathode.
U.S. patent application Ser. No. 07/975,791, filed Nov. 13, 1992, now abandoned, incorporated by reference herein in its entirety, describes and claims a fluid flow field plate for electrochemical fuel cells in which the inlet and outlet flow channels are discontinuous. The employment of discontinuous flow channels, as described in U.S. patent application Ser. No. 07/975,791, now abandoned, has several advantages:
1. Improved performance, particularly at higher reactant inlet pressures, resulting from (a) more effective water removal due to better access of the reactant stream to the electrocatalytically active region at the membrane/electrode interface, (b) more uniform current density due to more even distribution of the reactant stream across the electrocatalytically active area of the fuel cell and the avoidance of water pooling in the flow channels, and (c) lower flow field plate/electrode contact resistance due to the use of a decreased amount of the flow field plate surface to accommodate the flow channels.
2. Improved fuel cell lifetime resulting from (a) the ability to reduce the compressive load on the electrodes due to decreased contact resistance between the flow field plates and the electrodes, and (b) more uniform reactant gas relative humidity due to more even distribution of the reactant stream across the electrocatalytically active area of the fuel cell.
3. Reduced manufacturing costs resulting from (a) the ability to reduce the amount of graphite plate milling required for continuous channels and to relax the tolerances required for the channel dimensions, (b) a wider range of materials and fabrication techniques permitted with the discontinuous flow channel design, such as stamping of flow field stencils, to be employed, particularly the use of thinner electrically conductive sheet materials, as the discontinuous channels do not require the thickness and rigidity of the electrically conductive plates in which continuous, serpentine flow channels are formed, and (c) the ability to employ a stenciled graphite foil laminate, thereby reducing the weight (and cost) associated with rigid graphite flow field plates.
Conventional methods of fabricating fluid flow field plates require the engraving or milling of flow channels into the surface of the rigid, resin-impregnated graphite plates. These methods of fabrication place significant restrictions on the minimum achievable cell thickness due to the machining process, plate permeability, and required mechanical properties. For example, the minimum practical thickness for a double-sided flow field plate is approximately 0.075 inches.
The conventional resin-impregnated graphite plates are expensive, both in raw material costs and in machining costs. The machining of channels and the like into the graphite plate surfaces causes significant tool wear and requires significant processing times.
As described and claimed in U.S. patent application Ser. No. 08/024,660 (the "'660 application") now U.S. Pat. No. 5,300,370, incorporated herein in its entirety, fluid flow field plates may also be fabricated by a lamination process. Specifically, the '660 application discloses and claims a laminated fluid flow field assembly for an electrochemical fuel cell which comprises:
a separator layer formed of electrically conductive, substantially fluid impermeable sheet material, the separator layer having two oppositely facing major surfaces; PA1 a stencil layer formed of electrically conductive sheet material, the stencil layer having two oppositely facing major surfaces, the stencil layer having a fluid inlet formed therein and at least one opening formed therein extending between the major surfaces thereof, the at least one opening in fluid communication with the fluid inlet; and PA1 means for consolidating the separator layer and the stencil layer along one of their respective major surfaces. PA1 first and second embossed fluid flow field plates, each of the plates comprising: PA1 a membrane electrode assembly interposed between the first and second embossed fluid flow field plates, the membrane electrode assembly comprising: PA1 a sheet of compressible, electrically conductive, substantially fluid impermeable material, the sheet having two oppositely facing major surfaces, at least one of the major surfaces comprising an embossed surface, the embossed surface having at least one open-faced sealant channel embossed therein, the at least one embossed sealant channel circumscribing the central portion of the embossed surface and accommodating a substantially fluid impermeable sealant material therein, whereby the sealant material fluidly isolates the central portion from the atmosphere surrounding the plate, the embossed surface further having a coolant inlet, a coolant outlet, and at least one open-faced coolant channel formed therein, whereby the at least one coolant channel conducts pressurized fluid introduced at the coolant inlet toward the coolant outlet. PA1 a sheet of compressible, electrically conductive, substantially fluid impermeable material, the sheet having two oppositely facing major surfaces, at least one of the major surfaces comprising an embossed surface, the embossed surface having at least one open-faced sealant channel embossed therein, the at least one embossed sealant channel circumscribing the central portion of the embossed surface and accommodating a substantially fluid impermeable sealant material therein, whereby the sealant material fluidly isolates the central portion from the atmosphere surrounding the plate, the embossed surface further having a coolant inlet, a coolant outlet, and at least one open-faced coolant channel formed therein, whereby the at least one coolant channel conducts pressurized fluid introduced at the coolant inlet toward the coolant outlet. PA1 a sheet of compressible, electrically conductive material, the sheet having two oppositely facing major surfaces, at least one of the major surfaces comprising an embossed surface, the embossed surface having at least one open-faced sealant channel embossed therein, the at least one embossed sealant channel circumscribing the central portion of the embossed surface and accommodating a substantially fluid impermeable sealant material therein, whereby the sealant material fluidly isolates the central portion from the atmosphere surrounding the plate. PA1 a sheet of compressible, electrically conductive material, the sheet having two oppositely facing major surfaces and at least one manifold opening formed therein between the major surfaces, at least one of the major surfaces comprising an embossed surface, the embossed surface having at least one open-faced sealant channel embossed therein, the at least one embossed sealant channel circumscribing the at least one manifold opening and accommodating a substantially fluid impermeable sealant material therein, whereby the sealant material fluidly isolates the at least one manifold opening from the atmosphere surrounding the plate. PA1 providing a sheet of compressible, electrically conductive sheet material, the sheet having two oppositely facing major surfaces; PA1 embossing at least one open-faced channel in at least one of the major surfaces. PA1 forming a fluid inlet on at least one of the major surfaces such that the at least one channel extends from the fluid inlet, PA1 whereby the at least one channel conducts pressurized fluid introduced at the fluid inlet. PA1 forming a fluid inlet on at least one of the major surfaces; PA1 forming at least one open-faced channel in the at least one of the major surfaces, the at least one channel extending from the fluid inlet, whereby the at least one channel conducts pressurized fluid introduced at the fluid inlet.
In operation, the separator layer and the stencil layer cooperate to form at least one open-faced channel for conducting pressurized fluid introduced at the fluid inlet. The separator layer and the stencil layer are consolidated by compression, preferably in combination with an electrically conductive adhesive. The laminated fluid flow field assembly thus comprises two layers which must be properly positioned and aligned prior to consolidation.
It is often difficult and time consuming to properly position and align the separator and stencil layers of a laminated fluid flow field assembly. The two-layer laminated fluid flow field assembly also adds both volume and weight to the fuel cell, as compared, for example, to a one-layer fluid flow field assembly. Thus, as with conventionally fabricated fluid flow field plates, laminated fluid flow field assemblies restrict the extent to which cell thickness can be reduced because of the minimum thickness each plate or layer must possess to permit milling or engraving (in the case of graphite plates) or die-cutting (in the case of stencil layers or laminated assemblies).
Accordingly, it is an object of the present invention to provide an improved fluid flow field plate for use in electrochemical cells that is reduced in weight and volume, and that is simpler and less expensive to manufacture than conventional fluid flow field plates and laminated fluid flow field assemblies.
Another object of the present invention is to provide an improved fluid flow field plate for use in electrochemical cells that achieves a higher power density at a lower cost than conventional fluid flow field plates and laminated fluid flow field assemblies.
Still another object of the invention is to provide fluid flow field plates having improved sealing capabilities because of the presence of sealant grooves embossed in the surface of the plate.
A further object of the present invention is to provide improved coolant flow field plates for use in electrochemical cells.
A still further object of the invention is to provide an improved method of fabricating an embossed fluid flow field plate for use in electrochemical cells.